Alloy composition, method and apparatus therefor

ABSTRACT

Aspects of the disclosure are directed to forming a three-dimensional (3D) structure by depositing an alloy composition on a target, and solidifying portions of the alloy composition to form the 3D structure. The solidifying includes producing a martensitic structure by destabilizing a ferrite phase of the alloy composition while solidifying the alloy composition.

This invention was made with government support under 2011354 awarded bythe National Science Foundation.

BACKGROUND

Alloys are used for a multitude of structures, and may be formed in avariety of manners such as by casting, three-dimensional (3D) printing,and other approaches. However, for some applications it can bechallenging to produce a product having a desirable structure. Forinstance, when cooling metal alloys, various phases of material andcorresponding crystalline structures may form based on a variety offactors.

Stainless steel is an alloy used in a variety of applications. Forinstance, precipitation-hardened (PH) stainless steel known as 17-4 PH(otherwise known as AISI 630 or SAE Type 630) has a chemical makeup ofapproximately 15-17.5% chromium and 3-5% nickel, as well as 3-5% copper.17-4 PH stainless steel may be manufactured as a fully martensiticstainless steel via methods such as forging, casting or welding.However, additively manufactured 17-4 PH has a strong tendency ofreserving residual ferrite and austenite phases at room temperature uponsolidification, which may provide undesirable mechanical and corrosionperformance. For instance, in manufacturing with low cooling rates, 17-4PH undergoes phase transformations as: liquid (L)—δ-ferrite(δ)—austenite (γ)—martensite (α′). However, with additive manufacturing,transformation to fully martensite (e.g., at least 90% or at least 95%martensite) can be challenging. As such, it can be challenging to obtaindesired phases in materials built using additive manufacturing (AM) withmulti-stage phase transformations, because such materials may besensitive to complex AM processing conditions, such as rapidheating/cooling and chemical variation. For instance, 17-4 PH mayundergo different solidification paths during solidification in AMprocesses, which leads to the formation of different unwanted phases inthe printed parts.

It may also be challenging to obtain equiaxed grains in additivemanufacturing, especially in powder bed fusion. Due to the hightemperature gradient, columnar grains epitaxially grow from a previouslydeposited layer, along the build direction, and may be observed inadditive manufactured parts. The columnar grains are generally coarseand characterized by anisotropic mechanical properties, which may bedetrimental for applications involving multi-directional stresses.

These and other matters have presented challenges to the selection ofmaterials and formation of structures, for a variety of applications.

SUMMARY

Various example embodiments are directed to a composition of matter, itsimplementation and resulting apparatuses thereof. Such embodiments maybe useful for forming 3D structures, in a manner that addresseschallenges including those noted above.

As may be implemented in accordance with one or more embodiments, athree-dimensional (3D) structure is formed by depositing an alloycomposition on a target, and solidifying portions of the alloycomposition to form the 3D structure. The solidification includesproducing a martensitic structure by destabilizing a ferrite phase ofthe alloy composition while solidifying the alloy composition.

Another embodiment is directed to a method for additively manufacturinga three-dimensional (3D) structure. A first layer of the 3D structure isformed by depositing alloy powder on a target, liquefying the alloypowder via application of laser energy, and solidifying the liquefiedalloy powder to produce a martensitic structure by destabilizing aferrite phase of the alloy powder. Subsequent layers of the 3D structureare formed over the first layer by, for each subsequent layer,depositing additional alloy powder of the same composition of the alloypowder used in forming the first layer, liquefying the additional alloypowder via application of laser energy, and also solidifying theliquefied additional alloy powder to produce a martensitic structure bydestabilizing a ferrite phase of the additional alloy powder.

Another embodiment is directed to an alloy powder composition of matterfor forming martensitic 17-4 stainless steel via laser powder bed fusionadditive manufacturing. The alloy powder comprises iron, chromium,nickel, copper, niobium, and tantalum.

The above discussion/summary is not intended to describe each embodimentor every implementation of the present disclosure. The figures anddetailed description that follow also exemplify various embodiments.

BRIEF DESCRIPTION OF FIGURES

Various example embodiments may be more completely understood inconsideration of the following detailed description and in connectionwith the accompanying drawings, in which:

FIG. 1 shows a method for forming a structure, as may be implemented inaccordance with one or more embodiments;

FIG. 2 shows components of an alloy composition, in accordance withvarious embodiments, and with reference to 17-4 specification values aswell as 3D-printed values;

FIGS. 3A, 3B, 3C and 3D show phase transformation behavior of an alloyas may be implemented in accordance with one or more embodiments, inwhich:

FIG. 3A shows a phase constitution after cooling to room temperature,

FIGS. 3B and 3C show X-ray diffraction results showing the phasetransformation behavior, and

FIG. 3D shows an enhanced view of a portion of FIG. 3C; and

FIGS. 4A-4B show a scan approach as may be implemented to achieveas-built equiaxed fine grains utilizing an alloy composition ascharacterized herein, as may be implemented in accordance with one ormore embodiments, in which:

FIG. 4A depicts scan patterns for four stacked layers, and

FIG. 4B depicts an example scan pattern for one of the layers shown inFIG. 4A, with hatch spacing.

While various embodiments discussed herein are amenable to modificationsand alternative forms, aspects thereof have been shown by way of examplein the drawings and will be described in detail. It should beunderstood, however, that the intention is not to limit the invention tothe particular embodiments described. On the contrary, the intention isto cover all modifications, equivalents, and alternatives falling withinthe scope of the disclosure including aspects defined in the claims. Inaddition, the term “example” as may be used throughout this applicationis by way of illustration, and not limitation.

DETAILED DESCRIPTION

Aspects of the present disclosure are believed to be applicable to avariety of different types of articles of manufacture, apparatuses,systems and methods involving the formation of 3D structures, as mayinclude additive manufacturing, and which may involve what is oftenreferred to as 3D printing and/or casting. In certain implementations,aspects of the present disclosure have been shown to be beneficial whenused in the context of forming 3D metal structures using a compositionof materials that facilitate a near fully or fully martensiticstructure. While not necessarily so limited, various aspects may beappreciated through a discussion of examples using such exemplarycontexts.

According to various example embodiments, a liquid alloy includes acomposition of matter that operates to destabilize a ferrite phase ofthe alloy composition while it is solidified. This may involve, forinstance, transforming δ-ferrite to austenite and otherwise mitigatingthe formation of δ-ferrite phase material in a solidified form of thealloy. Substantially all of the austenite may further be transformed tomartensite, for instance such that the volume of the final structure is90-100% martensite.

The liquid alloy may be provided, for example, as part of a casting or3D printing operation, and may be formed by liquefying a powder alloycomposition. In this context, an alloy composition may bedeposited/provided on a target, and portions of the alloy compositionmay be solidified to form a 3D structure while destabilizing the ferritephase to product a martensitic structure. In some instances, laserpowder-bed fusion is utilized to effect laser melting successive layersof the alloy composition in powder form based on a 3D computer aideddesign (CAD) model.

Destabilizing the ferrite phase as characterized herein may be carriedout in a variety of manners. In some embodiments, the alloy includes acomposition of materials that reduces a temperature at which δ-ferritephase material forms during cooling of the alloy, relative to atemperature at which δ-ferrite phase material forms during cooling of a17-4 PH alloy composition. In other embodiments, the alloy includes acomposition of materials that increases a temperature at which δ-ferritephase material, which is formed from the alloy, transitions toγ-austenite phase material during cooling of the alloy, relative to atemperature at which δ-ferrite phase material transitions to γ-austeniteduring cooling of a 17-4 PH alloy composition. In certain embodiments,the composition of materials in the alloy are selected to effect both ofthe aforementioned aspects relating to reducing the temperature at whichδ-ferrite phase material forms and to increases the temperature at whichthe δ-ferrite material transitions to γ-austenite. For each of theseembodiments, the reference 17-4 PH alloy referred to as being relativeto the δ-ferrite transitions may refer to that formed from anArgon-atomized powder 17-4 PH composition, which is liquefied as it isdeposited in an additive manufacturing procedure.

In connection with particular embodiments, it has beenrecognized/discovered that an increase of Ni and Cu in a 17-4 PHcomposition can shorten the lifetime of δ-ferrite, promote the earlierformation of austenite, and reduce the δ-ferrite fraction duringsolidification. It has been further recognized that, compared withincreasing Ni and Cu, increasing Cr exhibits the opposite trend. Mn maybe excluded from the composition to mitigate high temperaturevolatility, and C and Si may be excluded from the composition tomitigate a drop of Ms temperature (the temperature at which martensitebegins to form). The presence of impurity type elements such as O, N, P,and S may be maintained as low as possible.

In certain embodiments, an alloy composition having materials consistingof iron, chromium, nickel, copper, niobium and tantalum is provided(e.g., deposited), and the composition of the respective materials isused to destabilize the ferrite phase of the alloy composition as it issolidified. For instance, a powder form of the alloy composition may bedeposited, liquefied with a laser, and solidified to form a 3Dmartensitic structure as characterized herein.

In some implementations, the alloy composition is as follows: Fe: 74.7%,Cr: 15.2%, Ni: 4.8%, Cu: 5.0% and Nb: 0.3%. This composition is utilizedto achieve the δ-ferrite phase transition temperatures as noted abovewith an additive manufacturing process, with a lattice parameter of theδ phase being 0.29403 nm upon initial solidification, relative to alattice parameter of 0.29437 nm 17-4 PH, using an atomized Argon powderdeposition with laser heating. Accordingly, the δ phase forms at a lowertemperature such that its formation is delayed via the alloycomposition, relative to δ phase formation in the 17-4 PH composition.The γ phase lattice parameter in the alloy composition is 0.36873 nm,whereas the Argon-atomized 17-4 PH lattice parameter is 0.36748 nm.Accordingly, the δ-γ transfer temperature in the alloy composition ishigher than that of the 17-4 PH composition, which permits more time forthe δ-γ transformation process to complete.

Turning now to the figures, FIG. 1 shows a flow diagram, as may beimplemented in accordance with one or more embodiments. At block 100, analloy composition is provided, such as by depositing and melting analloy composition powder. At block 110, the alloy composition is meltedor otherwise liquefied, such as by applying a laser to deposited powderas noted above. Certain approaches involve casting the alloy, in whichblocks 100 and 110 are utilized to provide a melted alloy into a cast.At block 120, the alloy composition melt is solidified while mitigatingthe formation of δ-ferrite as characterized herein. When utilized in anadditive manufacturing procedure, operations at blocks 100, 110 and 120are repeated, as represented at 130.

Block 120 may further include facilitating transformation of a liquidphase provided at block 110 to δ-ferrite, transformation of theδ-ferrite to γ-austenite, and transformation of the γ-austenite to α′martensite, utilizing the composition provided at block 100 to lower atemperature at which δ-ferrite forms, to increase a temperature at whichδ-ferrite transitions to γ-austenite, and to increase a temperature atwhich γ-austenite transforms to a′ martensite. This is depicted atblocks 121, 122 and 123.

Various approaches may be implemented in connection with the methoddepicted in FIG. 1. In connection with a particular embodiment, in-situlaser-melting is performed with a composition of material of: Fe(74.7%), Cr (15.2%), Ni (4.8%), Cu (5.0%), and Nb (0.3%).Destabilization of δ-ferrite is achieved, as indicated by a short-livedδ-ferrite peak as well as complete δ (110)-γ (111) transformation. Onthe other hand, the γ-α′ (austenite-martensite) transformation is alsocompleted, with a high Ms temperature of ˜170° C. (characterized by alattice parameter of 0.28800 nm), which may be achieved via theexclusion of certain minor elements sometimes included in 17-4 PH. Inthis manner, fully martensite may be achieved at room temperature. Thecooling rate in the sample may be increased from 1.88×10⁴° C./s to8.66×10⁴° C./s and achieve fully martensite, with nearly identicalsolidification behavior in which a primary-solidified short-livedδ-ferrite transforming completely into austenite, and ending with acomplete γ-α′ transformation with a high Ms temperature in the range of150° C.-190° C. Such a complete γ-α′ transformation may also be achievedwhen impurities are introduced (e.g., during a manufacturing process),such as may result in increased O and N concentration.

In accordance with a particular embodiment, it has beenrecognized/discovered that a composition of matter having parameters inthe ranges noted for the Alloy depicted in FIG. 2 can produce as-builtfully martensitic structure through additive manufacturing, bymitigating formation of a stabilized δ-ferrite phase, and mitigatingformation of a stabilized austenite phase. This recipe can be used for awide range of additive manufacturing conditions. FIG. 2 further depictsexemplary comparisons to specified and 3D-printed 17-4 PH compositionsthat may utilize an Argon-atomized powder that is melted via laserenergy and subsequently solidified.

In connection with various embodiments utilizing compositions ascharacterized herein, it has been recognized/discovered that fullymartensitic 17-4 PH can be built via additive manufacturing, with anincrease of martensite fraction along with decreased residual δ-ferritefraction in a final structure under elevated cooling rates.Surprisingly, the higher cooling rate leads to less residual δ-ferrite,with destabilized primary-solidified δ-ferrite such that a δ-γtransformation has a higher degree of completion. An increased austenitestart temperature, denoted by a larger lattice parameter, has beenrecognized/discovered under elevated cooling rates, via stimulation ofincreased cooling rate on an earlier δ-γ transformation. Alsosurprisingly, additional martensite was achieved under elevated coolingrates, which may be a consequence of reduced residual δ-ferrite. It hasfurther been recognized/discovered that a lower concentration of Cr anda higher concentration of Ni and Cu in a 17-4 PH alloy promotes earlierformation of austenite while reducing the stability of δ-ferrite.

The alloy composition may be deposited in a variety of manners. Forinstance, a liquid form of the alloy may be deposited and transformed toferrite, the ferrite then being transformed to austenite, and theaustenite then being transformed to a martensitic structure. The alloycomposition may be deposited as a powder with a laser being used to formliquid from the powder, the liquid being solidified thereafter while aferrite phase of the liquid is destabilized. Furthermore, layers may beformed by depositing, melting and solidifying additional powder of thealloy composition over a solidified or partially-solidified portion ofthe alloy composition.

In some embodiments, an alloy composition consisting of iron, chromium,nickel, copper, niobium and tantalum is utilized. For instance, a powderconsisting of this alloy composition may be deposited and melted vialaser, then solidified while destabilizing a ferrite phase of thesolidifying melt. In particular embodiments, the alloy compositionconsists of: 74-76% iron, 15-15.5% chromium, 4.5-5% nickel, 4.5-5%copper, and 0.15-0.45% of a mixture of niobium and tantalum, by weight.In other embodiments, the alloy composition consists of: 72.5-78.5%iron, 14.5-16% chromium, 4-5.5% nickel, 3-5.5% copper, and 0.15-0.45% ofa mixture of niobium and tantalum, by weight. It has beenrecognized/discovered that each of these alloy compositions mayfacilitate formation of a structure (e.g., additively manufactured)exhibiting substantially all martensite as noted herein. Certain furtherembodiments exhibit additional minor elements, such as one or more ofthe following: 0-1.5% manganese, 0-0.1% carbon, 0-1.5% silicon, 0-0.1%oxygen, 0-0.15% nitrogen, 0-0.04% phosphorous, and 0-0.03% sulfur, byweight.

A martensitic structure as characterized herein may be produced usingvarious approaches. For instance, the martensitic structure may beproduced via solidification while continuing to deposit more of thealloy composition thereon. The resulting structure may include a 3Dstructure having a volume of which at least 90% is martensite, or ofwhich at least 95% is martensite. Certain embodiments are directed tosolidifying portions of the alloy composition to form the 3D structureby forming precipitation hardening martensitic stainless steel havingequiaxed grains of a grain size that is less than 10 μm, and in certainembodiments less than 5 μm. It has been recognized/discovered that sucha grain size can be achieved, using the alloy compositions andapproaches characterized herein.

Another embodiment is directed to a method for additively manufacturinga three-dimensional (3D) structure. A first layer of the 3D structure isformed by depositing alloy powder on a target, liquefying the alloypowder using a laser, and solidifying the liquefied alloy powder toproduce a martensitic structure while destabilizing a ferrite phase ofthe alloy powder. Subsequent layers of the 3D structure may be similarlyformed over the first layer. For each subsequent layer, additional alloypowder of the same composition is deposited, the additional alloy powderis liquefied using the laser, and the liquefied additional alloy powderis solidified to produce a martensitic structure while destabilizing aferrite phase of the additional alloy powder.

Destabilizing the ferrite phase in this context may include usingapproaches as characterized above. For instance, a composition of thealloy powder can be set to reduce a temperature at which δ-ferrite phasematerial forms during cooling of the liquefied alloy powder, relative toa temperature at which δ-ferrite phase material forms during cooling ofa liquefied 17-4 PH alloy. The composition of the alloy powder may beset to increase a temperature at which δ-ferrite phase materialtransitions to γ-austenite phase during cooling of the liquefied alloypowder, relative to a temperature at which δ-ferrite phase materialtransitions to γ-austenite during cooling of a liquefied 17-4 PH alloy.Both of the aforementioned reduction and increase in temperature may beobtained with a particular alloy composition (e.g., a powder consistingof iron, chromium, nickel, copper, niobium and tantalum may be used).

FIGS. 3A, 3B, 3C and 3D show phase transformation behavior of an alloy,as may be implemented in accordance with one or more embodiments.Beginning with FIGS. 3B and 3C, X-ray diffraction results are depictedrespectively for cooling times corresponding to 0-1 seconds in FIG. 3C,and 1-19 seconds in FIG. 3B. FIG. 3D shows an enhanced view of inset 301of FIG. 3C. FIG. 3D shows that, when solidification begins, δ-ferrite331 at 301 exists for a very short time, upon which the material becomesγ-austenite 321. This is also shown in FIG. 3C with δ-ferrite at 331,332 and 333 transitioning to γ-austenite at 321, 322 and 323.

Referring to FIG. 3B, the γ-austenite 321, 322, 323 lasts for nearly 3seconds before transforming into α′ martensite 310, 311 and 312. Thefinal phase may be fully or nearly fully α′ martensite. Referring backto FIG. 3A, shows a phase constitution of α′ martensite 310, 311 and312, as may be present after cooling to room temperature. Accordingly,such a resulting structure can be realized using compositions andrelated approaches as recognized/discovered and noted herein.

FIGS. 4A-4B show a scan pattern as may be implemented to achieveas-built equiaxed fine grains utilizing an alloy composition ascharacterized herein, as may be implemented in accordance with one ormore embodiments. FIG. 4A depicts respective scan patterns as may beutilized to form successive layers, with each successive layer utilizinga scan pattern that is orthogonal relative to another or other layersimmediately above and/or below the layer. FIG. 4B depicts one of thescan patterns of FIG. 4B with hatch spacing 400 (e.g., 0.05-0.2 mm).

Various embodiments are directed to achieving equiaxed fine grains inadditive manufacturing of a designed alloy via in-processheat-treatment. A continuous-wave fiber laser with a wavelength of1070±10 nm, with a spot size of 150˜300 μm and 300˜500 W power may beused with a scan speed of 0.05˜0.35 m/s. Feedstock powder supplied forforming an apparatus may be gas-atomized with argon or nitrogen, with acomposition as characterized in one or more embodiments herein. Multiplelayers may be formed (e.g., as shown in FIG. 4A), with a spreadingthickness of each layer being 35˜150 μm. In a more particularimplementation, the laser spot size is set 230 μm, the laser power isset to 500 W, and the laser scan speed is set to 0.11 m/s, with a scanpattern that alternates 90 degrees for each layer (e.g., as shown inFIG. 4A, and with hatch spacing as shown in FIG. 4B being 0.05-0.2 mm).

Using this approach, it has been recognized/discovered that an as-builtpart may be fully composed of equiaxed fine grains with an average grainsize of ˜3 μm. Columnar grains and epitaxial growth are mitigated orprevented in the as-built part. Accordingly, an alloy composition and AMprocessing conditions as characterized herein can be utilized to achieveequiaxed fine grains in an as-built AM part by controllingphase-transformation dynamics via in-process heat-treatment. It hasfurther been recognized/discovered that forming a 3D structure inaccordance with these approaches may be implemented by formingprecipitation hardening martensitic stainless steel having equiaxedgrains of a grain size that is less than 10 μm. Certain embodiments aredirected to such an approach, and embodiments are directed to astructure having such equiaxed grains and related grain size, which hasbeen unexpectedly attained via use of alloy compositions and relatedapproaches herein.

Based upon the above discussion and illustrations, those skilled in theart will readily recognize that various modifications and changes may bemade to the various embodiments without strictly following the exemplaryembodiments and applications illustrated and described herein. Forexample, the percentages of respective components in the alloyscharacterized herein may be varied while maintaining therecognized/discovered improvements relative to formation of a fullymartensitic structure, such as those increasing the transitiontemperature of δ-ferrite to γ-austenite, and of γ-austenite to α′martensite. Further, approaches herein may utilize the compositionscharacterized in a variety of manufacturing approaches, includingadditive manufacturing as well as casting. Various additivemanufacturing procedures may be utilized as well, including laser powderbed fusion as noted herein, as well as blown powder additivemanufacturing, wire-feed additive manufacturing, and foil-feed additivemanufacturing. Further, different forms of product such as powder, wire,and foil may be provided in accordance with such applications. Suchmodifications do not depart from the true spirit and scope of variousaspects of the invention, including aspects set forth in the claims.

What is claimed is:
 1. A method for forming a three-dimensional (3D)structure, the method comprising: depositing an alloy composition on atarget; and solidifying portions of the alloy composition to form the 3Dstructure, including producing a martensitic structure by destabilizinga ferrite phase of the alloy composition while solidifying the alloycomposition.
 2. The method of claim 1, wherein destabilizing the ferritephase includes facilitating transformation of δ-ferrite to austenite andmitigating the formation δ-ferrite phase material from the alloycomposition.
 3. The method of claim 2, wherein solidifying the portionsof the alloy composition includes transforming substantially all of theaustenite to martensite.
 4. The method of claim 1, wherein destabilizingthe ferrite phase includes depositing the alloy composition withmaterials that reduce a temperature at which δ-ferrite phase materialforms from the alloy composition during cooling thereof, relative to atemperature at which δ-ferrite phase material forms during cooling of a17-4 PH alloy composition.
 5. The method of claim 1, whereindestabilizing the ferrite phase includes depositing the alloycomposition with materials that increase a temperature at whichδ-ferrite phase material formed from the alloy composition transitionsto a γ-austenite phase material during cooling thereof, relative to atemperature at which δ-ferrite phase material transitions to γ-austeniteduring cooling of a 17-4 PH alloy composition.
 6. The method of claim 1,wherein destabilizing the ferrite phase includes depositing the alloycomposition with materials that: reduce a temperature at which δ-ferritephase material forms from the alloy composition during cooling thereof,relative to a temperature at which δ-ferrite phase material forms duringcooling of a 17-4 PH alloy composition; and increase a temperature atwhich δ-ferrite phase material formed from the alloy compositiontransitions to a γ-austenite phase material during cooling thereof,relative to a temperature at which δ-ferrite phase material transitionsto γ-austenite during cooling of a 17-4 PH alloy composition.
 7. Themethod of claim 6, wherein the 17-4 PH alloy composition includesArgon-atomized powder.
 8. The method of claim 6, wherein producing themartensitic structure by destabilizing a ferrite phase of the alloycomposition while solidifying the alloy composition includes formingaustenite and transforming substantially all of the austenite tomartensite.
 9. The method of claim 1, wherein depositing the alloycomposition includes depositing an alloy composition consisting of:iron, chromium, nickel, copper, niobium and tantalum.
 10. The method ofclaim 1, wherein destabilizing the ferrite phase of the alloycomposition while solidifying the alloy composition includes depositingthe alloy composition having materials consisting of iron, chromium,nickel, copper, niobium and tantalum, and using the composition of therespective materials to destabilize the ferrite phase thereof.
 11. Themethod of claim 1, wherein depositing the alloy composition includesdepositing an alloy composition consisting of: 72.5-78.5% iron, 14.5-16%chromium, 4-5.5% nickel, 3-5.5% copper, and 0.15-0.45% of a mixture ofniobium and tantalum, by weight.
 12. The method of claim 1, whereinproducing the martensitic structure via the solidifying includesproducing the martensitic structure while continuing to deposit more ofthe alloy composition thereon.
 13. The method of claim 1, whereinproducing the martensitic structure via the solidifying includessolidifying all of the deposited alloy composition to form the 3Dstructure.
 14. The method of claim 1, wherein: depositing the alloycomposition on a target includes depositing a liquid form of the alloy;and producing the martensitic structure via the solidifying includestransforming the liquid alloy to ferrite, transforming the ferrite toaustenite, and transforming the austenite to the martensitic structure.15. The method of claim 1, wherein producing the martensitic structureincludes forming the 3D structure having a volume of which at least 90%is martensite.
 16. The method of claim 1, wherein: depositing the alloycomposition includes depositing the alloy composition as a powder; andsolidifying the portions of the alloy composition includes forminglayers of the alloy composition by using a laser to form liquid from thepowder and thereafter solidifying the liquid while destabilizing aferrite phase of the liquid, including melting and solidifying powder ofthe alloy composition that is deposited over a solidified portion of thealloy composition.
 17. The method of claim 1, wherein solidifying theportions of the alloy composition to form the 3D structure includesforming martensitic stainless steel having equiaxed grains of a grainsize that is less than 10 μm.
 18. A method for additively manufacturinga three-dimensional (3D) structure, the method comprising: forming afirst layer of the 3D structure by depositing alloy powder on a target,liquefying the alloy powder via application of laser energy, andsolidifying the liquefied alloy powder to produce a martensiticstructure by destabilizing a ferrite phase of the alloy powder; formingsubsequent layers of the 3D structure over the first layer by, for eachsubsequent layer, depositing additional alloy powder of the samecomposition of the alloy powder used in forming the first layer,liquefying the additional alloy powder via application of laser energy,and solidifying the liquefied additional alloy powder to produce amartensitic structure by destabilizing a ferrite phase of the additionalalloy powder.
 19. The method of claim 18, wherein forming the firstlayer and forming the subsequent layers include: destabilizing theferrite phase using a composition of alloys in the alloy powder toreduce a temperature at which δ-ferrite phase material forms duringcooling of the liquefied alloy powder, relative to a temperature atwhich δ-ferrite phase material forms during cooling of a liquefied 17-4PH alloy; and increasing a temperature at which δ-ferrite phase materialtransitions to γ-austenite phase during cooling of the liquefied alloypowder, relative to a temperature at which δ-ferrite phase materialtransitions to γ-austenite during cooling of a liquefied 17-4 PH alloy.20. The method of claim 18, wherein the alloy powder consists of: iron,chromium, nickel, copper, niobium and tantalum.
 21. An alloy powdercomposition of matter for forming martensitic 17-4 stainless steel vialaser powder bed fusion additive manufacturing, the alloy powdercomprising: iron; chromium; nickel; copper; niobium; and tantalum.